Clean up undefined behaviour

[Ryan-rsm-McKenzie]

This pr closes #32

The issue is is that output is borrowed exclusively here:

realfft/src/lib.rs

Lines 902 to 906 in ed26ea3

	let buf_out = unsafe {
	let ptr = output.as_mut_ptr() as *mut Complex<T>;
	let len = output.len();
	std::slice::from_raw_parts_mut(ptr, len / 2)
	};

But is then used again in the expression output.len() / 2 while it is still borrowed:

realfft/src/lib.rs

Lines 907 to 908 in ed26ea3

	self.fft
	.process_outofplace_with_scratch(&mut input[..output.len() / 2], buf_out, scratch);

[HEnquist]

Thanks for the PR! I'll make a new release as soon as I have a little time.